1. Field of the Invention
The present invention is directed to strapping machines and, more particularly to strapping machines for strapping loads fed in a back-to-back manner.
2. Background of the Invention
Strapping machines are in widespread use for applying a strap, such as a plastic strap, in a tensioned loop around a load. A typical strapping machine includes a strap chute for guiding the strap around the load, a strapping head through which the leading end of the strap is fed, and at which the strap is tensioned and secured to itself, and a strap dispenser to dispense a desired length of strap from a strap material supply.
In one application, the strapping machine is used to strap bundles of printed materials. For example, newspapers are often bundled and strapped for subsequent handling and transportation.
It is of course most efficient to bundle a series of loads in an automatic operation. Preferably, such an operation is fully automatic to alleviate the need for handling loads as they progress along a production line. In many such machines, conveyors and belts move the loads into and out of the strapping machine.
In an operation in which the loads approach the strapping machine in discrete, separated, and spaced-apart groups, such operation is readily accomplished. For example, a switch on the machine prior to the strap chute senses the presence or absence of a load. Once the switch is activated by the presence of the load on the switch and released by the passage of the load over the switch, a timing circuit actuates to measure a predetermined time at which the conveyor will stop to allow the strapping operation to proceed. The strapped load is then conveyed away from the chute and a similar cycle is commenced for the next load.
While this system functions well for loads or bundles of material that are separated and well spaced from one another, it has been found to have its drawbacks when bundles of material are disposed in a back-to-back progression, wherein the bundles are immediately next to one another or very closely spaced. For example, the bundles may be improperly strapped or not at all strapped when the bundles are so close that the switch remains depressed through multiple cycles or when the bundles are so close that the timing circuit does not correspond to bundle spacing. Generally, 3 to 4 bundles will be properly strapped (by virtue of the timing circuit being close in determining when to apply the next course of strap), but after about 3-4 bundles the timing is off. This results in misfed strap and requires the strapping machine to be shut down, the bundles realigned, and the system restarted. As such, it is often necessary to have an operator positioned at or near the machine at most times to assure that the bundles are properly positioned to enter the strapper and are properly strapped when leaving.
To avoid these problems, prior strapping machines have been fitted mechanical switches and optical sensors to determine when the loads entered the strapping machine arch area (at the strap chute) and when the loads exited the arch. Here, however, problems arose with loads contacting and actually striking bundle stop arms that were part of the machines. In addition, problems arose in situations where, for example, a single sheet of material, for example, a newspaper insert, was misaligned with the rest of the bundle, and the system could not detect when the bundle was in proper position for strapping or for conveyance out of the strapping machine arch.
Accordingly, there is a need for a strapping machine that can be used in back-to-back and normal spaced-apart bundle operations that will accurately control movement of the bundles and strapping thereof.